Postnatal decrease in muscarinic cholinergic influence on Ca2+ currents of rabbit ventricular cells.

We have studied developmental changes in the muscarinic cholinergic modulation of L-type Ca2+ current (ICa) in enzymatically isolated adult and newborn (1-4 days old) rabbit ventricular cells using the whole-cell patch-clamp method. Carbachol (10 microM) caused a 2.8-fold increase in the half-maximal concentration (EC50) for isoproterenol to stimulate ICa for adult cells compared with the control with little effect on the maximal ICa density (Imax), whereas the stimulatory effect of isoproterenol on newborn ICa was completely eliminated by 10 microM of carbachol and was decreased by 40% at 0.1 microM carbachol. Carbachol increased the EC50 for forskolin to stimulate ICa 2.9-fold for adult cells and 7.3-fold for newborn cells with little effect on the Imax for either group. 5'-Guanylyl imidodiphosphate [Gpp(NH)p; 100 microM] reduced the stimulatory effect of 0.1 microM isoproterenol on adult ICa (percent increase over predrug level) by a factor of 1.8 compared with the control (400 microM guanosine triphosphate), whereas the isoproterenol effect on newborn ICa was completely eliminated by Gpp(NH)p. The isoproterenol effect on adult ICa persisted after the washout of isoproterenol in the presence of Gpp(NH)p. Gpp(NH)p also reduced the stimulatory effect of 1 microM of forskolin by a factor of 2.0 and 8.2 for adult and newborn cells, respectively, in comparison to the control. Carbachol caused no additional effect on forskolin-stimulated ICa for either adult or newborn ICa in the presence of Gpp(NH)p. Pretreatment with pertussis toxin completely eliminated the inhibitory effect of carbachol on forskolin-stimulated ICa for both groups. In addition, the effect of forskolin on ICa was markedly enhanced by the pertussis toxin pretreatment in newborn cells, whereas the enhancement was relatively small for adult cells. We conclude that the muscarinic cholinergic influence on L-type ICa decreases after birth in rabbit ventricular cells, presumably through a diminishing influence of an inhibitory G protein on regulating adenylyl cyclase during the postnatal period.